1. Field of the Invention
The present invention relates to methods for electrically heating subsea pipelines. More particularly, the invention relates to joining and repair of insulation on a single heated insulated pipe.
2. Description of the Related Art
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep and where temperatures of the water near the sea floor are in the range of 40.degree. F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid material, called a "hydrate." Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressuring the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography require excessive time and may create more operational problems and be costly in terms of lost production.
The problem of lower temperatures in subsea pipelines has been addressed by placing thermal insulation on the lines, but the length of some pipelines makes thermal insulation alone ineffective. Increased flow rate through the lines also helps to minimize temperature loss of the fluids, but flow rate varies and is determined by other factors. Problems of heat loss from a pipeline increase late in the life of a hydrocarbon reservoir because production rates often decline at that time. Problems become particularly acute when a pipeline must be shut-in for an extended period of time. This may occur, for example, because of work on the wells or on facilities receiving fluids from the pipeline or hurricane shut-downs. The cost of thermal insulation alone to prevent excessive cooling of the lines becomes prohibitive under these conditions.
Heating of pipelines by bundling the lines with a separate pipeline that can be heated by circulation of hot fluids has been long practiced in the industry. Also, heating by a variety of electrical methods has been known. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. ("Direct Impedance Heating of Deepwater Flowlines," OTC 11037, May, 1999).
Two configurations for heating have been considered. In one method of electrical heating, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is the inner pipe and it is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at one end. Voltage is applied at the opposite end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in Ser. No. 08/625,428, filed Mar. 26, 1996, which is commonly assigned.
In a second configuration, a single flowline is electrically insulated and current flows along the flowline. This is called the "SHIP" system (Single Heated Insulated Pipe). Two SHIP systems have been considered: the fully insulated system, requiring complete electrical insulation of the flowline from the seawater, and the earthed-current system, requiring electrical connection with the seawater through anodes or other means. For both systems, current is passed through the flowline pipe.
An earthed-current system developed in Norway does not require the use of insulating joints or defect-free insulation, and hence avoids the problem of shorting by water and the effects of coating defects. ("Introduction to Direct Heating of Subsea Pipelines," overview by Statoil, Saga et al, February 1998). In that system, power is connected directly to the pipe at each end of a heated section and electrodes corrected to the pipe along the pipeline are exposed to seawater. This configuration allows current to flow in both the pipe and the seawater, therefore eliminating potential drop across the insulation on the pipe, so that a defect in the pipe insulation does not result in electrical failure. Since the heated section is not electrically isolated from the rest of the pipeline by insulation joints, some means must be provided to prevent current from flowing along the pipeline to areas where it may cause corrosion damage or interfere with control systems. This is accomplished by means of a buffer zone, which is a length of pipe approximately 50 meters in length between the power connection where current enters or leaves the pipeline and adjacent structures. In that design, the buffer zone may incorporate a series choke to further impeded leakage currents. This method requires that the return cable be as close to the pipe as possible, or electrical efficiency will be impractically low. This configuration is not practical for some deepwater applications and the system is considerably less energy-efficient than a fully insulated system.
A fully insulated method of electrically heating a pipeline is disclosed in U.S. Pat. No. 6,049,657. In this method, an electrically insulated coating covers a single pipeline carrying fluids from a well. An alternating current is fed to one end of the pipeline through a first insulating joint near the source of electrical current and the current is grounded to seawater at the opposite end of the pipe to be heated through a second insulating joint.
The SHIP method of electrical heating of pipelines offers many advantages, but suffers from the significant disadvantage that any defects in the insulation would allow seawater to reach the pipe, which would accelerate corrosion of the pipe. Therefore, there is a need for methods for fabricating and repairing thermoplastic layers of insulation, especially at field joints where sections of pipe are welded together, so that the pipe can be joined and the insulation be maintained defect-free.